I have been asking him for proof that Muhammad was a prophet. At the same time he has been calling on me to prove that Jesus was/is God.
As for the former, I have maintained that there is no proof that Muhammad was a prophet sent by God, insofar as there are no bona-fide predictions from the former prophets witnessing to Muhammad's coming, insofar as Muhammad made no accurate predictions himself that could only come from God, and insofar as Muhammad performed no miracles or otherwise pointed to any sign from God that is commensurate with proving the claim that "Muhammad is the Messenger of Allah". In short, I have maintained that there is no independent proof of Muhammad's prophethood that is consistent with the sorts of confirmatory signs that the former prophets came with, or some other proof that he can provide that cannot come from anyone other than God.
I have also shown that his case is greatly compounded by the fact that, although some prophets did not perform miracles, as he rightly pointed out, their coming was either prophesied or otherwise attested by God in some way, such as through another God-confirmed prophet, and, together with this, that the Qur'an itself obligates Muslims to find just such proof, i.e. proof in the prior Scriptures for Muhammad.
As for the latter challenge, his challenge to me to show that Jesus was/is God, I supplied many verses. Of course, he made the artificial restriction common to Muslims that confirmation of Christ's deity from the Scriptures had to be: 1) made by Jesus Himself not the prophets before him, or the apostle's who were commissioned to speak in His name (What gives, I thought Muslims believed in the previous prophets and made no distinction between any of them?); and 2) it had to be direct and explicit, i.e. it couldn't be the sort of thing where Jesus received or positively commended someone else for saying or responding to Him in a way only appropriate for deity.
Of course, I do not except this methodology, for I believe that the former prophets spoke from God, as did the apostle's, something Muslims (in their own muddled way) pay lip-service to but never carry out in their practice. Nevertheless, I have sought to be magnanimous and so have happily obliged my Muslim friend and submitted myself to these strictures for the time being.
Accordingly, my first proof that Christ Himself, in His own person, from His own lips, in a direct and explicit fashion, claimed to be God is from Revelation 1, where it is written: "7 BEHOLD, HE IS COMING WITH THE CLOUDS, and every eye will see Him, even those who pierced Him; and all the tribes of the earth will mourn over Him. So it is to be. Amen. 8 'I am the Alpha and the Omega," says the Lord God, "who is and who was and who is to come, the Almighty.'"
Regarding this passage, my Muslim friend has, as yet, made no comment. In the course of our conversation we have spoken of various and sundry other things as well, such as prostration in prayer, fasting, textual integrity of the Bible and the Qur'an and more. But the foregoing topics have been at the heart of our conversation and have taken up the bulk of our correspondence.
I wrote my Muslim friend and told him I was redirecting things here, and welcome him or any other Muslim to comment and help him out. I also welcome anyone else into this discussion who wishes to comment. My Muslim friend is also free, provided these topics are not skirted, to bring up any of the other subjects we have talked about or that he would like to initiate a discussion on.
I look forward to what he (or anyone else) has to say.
To the law and the testimony, if they speak not according to this word it is because there is no light in them (Isaiah 8).
And remember We gave Moses the Scripture and the Criterion (between right and wrong), there was a chance for you to be guided aright. (Qur'an, S. 2:53, Yusuf Ali)